The present invention relates to photosynthetic organisms and, compositions and methods of generating same.
The energy crises of the 1970's combined with world-wide climate changes linked to the accumulation of excess carbon dioxide in the atmosphere, has initiated a significant return to the development and use of biomass as a resource for fuel and chemical products. Presently, society's dependence is shifting away from petroleum to renewable biomass and energy resources, in order to aid in the development of a sustainable industrial society and to manage the green house effect. The US department of energy has set goals to replace 30% of the liquid petroleum transportation fuel with biofuels and to replace 25% of the industrial organic chemicals with biomass-derived chemicals by 2025 [Ragauskas et al, Science, 311, 484-489 (2006)]. The EU has set targets at 5.75% for all petrol and diesel transport fuels to be biomass derived by the end of 2010 [Ragauskas et al, supra].
Biomass production methods presently enable different chemical and biofuel (e.g. biodiesel and ethanol) production from plant resources. Current production of biomass in general and biofuel in particular mainly relies on higher plants and trees, as for example corn and sugar cane plants. However, biofuel production from these plants is limited in yield and enhanced consumption of such plants as a biofuel source would lead to a severe shortage in food supply worldwide. Furthermore, maximum productivities of higher plants and trees are restricted to areas with prime soil, water, and climate (primarily the tropics). Plant leaves exist in an aerial environment and are subject to large evaporative moisture losses, which directly inhibit the process of photosynthesis.
Biomass production by cyanobacteria and microalgae, which are the most productive carbon dioxide users of all photosynthetic organisms and can fix greater amounts of carbon dioxide per land area than higher plants, is restricted to a relatively narrow range of temperatures defined by their native habitat. Mesophilic organisms show maximal rate constant at 20-30° C. while thermophilic strains achieve similar rates at 60-70° C. This temperature range mainly reflects constrains of the photosynthetic energy conversion machinery (photosystems) and of the carbon fixation one (the Rubisco complex).
Biomass is mainly generated in the course of photosynthesis which photocatalyses carbon dioxide fixation via the Rubisco complex. Plants, microalgea and cyanobacteria use photosystems I and II (PSI and PSII), to convert light energy into chemical energy. The central unit of a PSII protein complex is the reaction center (RC). The functional core of PSII RC consists of a heterodimer made of the two homologous protein subunits D1 and D2 along with one unit of cytochrome b559. The D1 and D2 protein subunits each have five (A, B, C, D, and E) transmembranal (TM) α helices. The cofactors that carry out electron transfer (chlorophillus type molecules and quinones) in response to illumination and thereby perform the primary energy conversion, are mainly bound (non-covalently) to helices D and E of the D1/D2 subunits.
Numerous studies showed that the photosynthetic energy conversion by PSII RC is highly sensitive to irradiation and temperature variations [Takahashi et al., Plant Cell Physiol 45, 251-5 (2004); Murata et al Biochim Biophys Acta 1767, 414-21 (2007)]. Also, recent studies have suggested that the PSII RC is a key player in regulating the rate of photosynthetic energy conversion in response to the prevailing temperature [Yamasaki et al., Plant Physiol 128, 1087-97 (2002)].
Taking into consideration the global warming effect, annual and even daily changes in temperature in aquatic areas (e.g., oceans, small lakes and ponds), dramatically narrow the efficiency of biomass production of thermophilic and mesophilic microalgal strains as well as of different strains of cyanobacteria and thus limits their growth to the tropical arena. Even there, current global heating is expected to exceed the thermotolerance and production efficacy of these organisms [Wraight, Front Biosci 9, 309-37 (2004); Behrenfeld et al. Nature 444, 752-5 (2006)]. Although the expected changes in global temperatures are only in the order of several degrees, it is predicted that biomass production may be dramatically effected.
The increased need for biofuel and the concomitant shortage of food across the world underscores the urgent need for methods of increasing resistance of plants, algea and microalgae to ambient temperature changes.